This invention relates to coating formulations and a method, useful in microelectronics applications, for isolating and protecting fine-pitch, electrically conducting circuit interconnects, and related structures. More particularly the invention provides coating materials for application to conductive elements using an electrophoretic deposition technique. The coatings provide protective, high resistivity, low dielectric constant, negative image bearing layers after exposure to radiation patterns of suitable wavelength, followed by development with mild aqueous acid solutions.
Modern society relies upon the trouble-free conveniences provided by electrical and electronic devices. Since the earliest recognition that useful devices could be developed by combining electrical circuits, circuit combinations have become more complex, and the resulting devices more sophisticated in their capabilities. Effective circuit performance relies upon electrical current isolation within a particular circuit with no possibility of current leakage into a neighboring circuit. Any unintended current transfer between circuits of a multi-circuit, multi-function electrical device will ultimately cause an inconvenient malfunction of the device.
Isolation or insulation of circuits from each other represents an increasing challenge with the continuing emphasis on more complex printed circuit designs and increased functionality for electrical devices, especially miniature electronic devices. Progress in electrical device design has caused a transition from the interconnection of discrete electrical components, using pre-insulated wiring structures, to interconnection, with modern printed circuits, using conductive traces only microns wide. Protection and isolation of such narrow traces, from each other, demands materials that may be precisely placed over the elongate current carrying traces while leaving tiny contact points exposed for electrical connection to other circuits that form part of a particular device. For a significant period of time it was possible to essentially cover the printed circuit with a protective coating, leaving voids in the coating corresponding to the needed points of contact. More recently, however, the introduction of flexible printed circuits and multi-layer printed circuits has led to the need for coatings and processes capable of high precision in protective cover formation and placement. High precision techniques provide a cover-layer with essentially just sufficient insulation to protect a conductive trace without straying into other portions of a printed circuit substrate. Such coatings tend to be very thin and subject to attack by, e.g. solvents, moisture, or other potentially damaging environments. For this reason, precision coating of printed circuits must provide both insulative and environmental protection for electrical conductors.
A variety of coating methods exists for applying coatings, covercoats and the like as protective, insulating coatings to printed circuit patterns. The term covercoat refers to a dielectric coating, over the printed circuit basestock, applied after the conductive circuit pattern has been etched. The covercoat serves to protect the copper conductors from moisture, contamination and damage. Conventional coating methods include screen printing and application of continuous layers by methods such as knife coating, spin coating, extrusion coating, dip coating, curtain coating, and spray coating. Application of continuous coatings covers not only the leads but also the area in between the leads. This condition has several disadvantages when found in intricately structured printed circuits. For example, differences in expansion coefficients between a continuous cover-coat and a flexible printed circuit substrate may introduce stresses that cause the circuit to adopt an inconvenient curl-set. Segmentation of a cover-coat, into separate coated areas, is less likely to be subject to this condition.
Selective deposition processes, such as electrophoretic deposition, also known as xe2x80x9ce-coat,xe2x80x9d may achieve coating separation and precise positioning (details of this process may be found in the xe2x80x9cHandbook of Electropainting Technologyxe2x80x9d by W. Machu, Electrochemical Publication Limited, 1978). Application of electrophoretic deposition techniques began at least three decades ago for painting automobiles and appliances. Electrophoretic deposition involves precise distribution of a layer of charged droplets over a conducting surface that represents an electrode of an electrolytic cell operating under direct current potential. Charged droplets migrate towards an oppositely charged electrode to be deposited thereon. Droplet deposition and layer formation may occur at either an anode or a cathode. Preferably the droplets are positively charged for deposition on a cathodic surface. Cathodic coatings do not suffer the oxidative corrosive processes associated with anodic deposition. Also, electrophoretic deposition of water-based compositions produces essentially void free and substantially non-polluting coatings.
Compared to conventional coating processes, such as screen printing, electrophoretic deposition selectively places a protective layer only on conductive portions of the printed circuit. Use of electrophoretic deposition should produce individually encapsulated conductors, whereas conventional techniques coat the entire printed circuit. Selective deposition also offers other advantages, such as the production of lighter weight circuits which is important for hard disk drive (HDD) flexible circuits applications.
The use of electrophoretic deposition is known for coating printed circuits with photoresists. U.S. Pat. Nos. 4,845,012; 5,055,164; 5,607,818; 5,384,229; 5,959,859; and 5,439,774 contain reference to the technique. Other U.S. Pat. Nos. 4,592,816 and 5,181,984 describe epoxy/acrylate compositions for electrophoretic deposition of solder mask/covercoat systems. Photoresist and solder mask materials are typically photosensitive and developable to a patterned polymer, covering selected (imaged) portions of the printed circuit. This provides evidence of photoimageable coatings, formed by electrophoretic deposition. Additionally, U.S. Pat. No. 4,832,808 teaches electrophoretic deposition of coatings of piperazine-containing polyimides. However, such coatings possess neither photosensitivity nor solubilization in aqueous acid developers.
The effective use of electrophoretically deposited, photoimageable coatings may depend upon the image resolution attainable with such systems. Printed circuits of increasing density require the use of photoresists of increasing image resolution. Image resolution depends upon radiation scattering within photosensitive layers and the variation of image characteristics, i.e. resolution, related to developers and development processes.
Polyimide-containing formulations provide potentially useful materials for photoimageable coatings produced by electrophoretic deposition. They also have the thermal and dielectric properties suitable for protecting and insulating electrical current carrying conductors. Image development of polyimide coatings, after exposure to an image pattern, may involve non-aqueous, solvent-based developers or aqueous-based developers. The use of solvent-based development systems applies to photoimageable polyimides that may use a benzophenone moiety as a built-in photo-crosslinker. U.S. Pat. Nos. 4,629,685; 4,656,116; 4,841,233; 4,914,182; 4,925,912; 5,501,941; 5,504,830; 5,532,110; and 5,599,655; and European Patent No. EP 0456463 A2 provide evidence of autosensitized polyimides. As indicated previously, these materials need organic solvents for image development. High volume use of solvent developers, in production operations, may cause environmental problems associated with solvent pollution and disposal. Aqueous developers provide a more environmentally friendly alternative to organic solvent developers. Some alkaline aqueous developers contain tetramethylammonium hydroxide as an agent for image development of photoimageable polyimides derived from either polyamic acid or phenolic derivatives. These precursors tend to produce polyimides having residual reactivity, leading to copper oxide formation, when deposited on copper, along with related corrosion of metallic copper that could result in poor coated film properties.
Considering the disadvantages of previously discussed, solvent-based and alkaline aqueous image developers and the benefits of selective coating deposition processes, there is a need for electrophoretically deposited, photoimageable polyimide coatings, soluble in non-polluting, preferably aqeous image developers.
The present invention provides photoimageable polyimide coatings applied from emulsion or solution formulations using electrophoretic deposition techniques. Such coatings function as image recording materials through exposure to a pattern of suitable radiation. An image, formed in a coating according to the present invention, may be revealed using an acidified aqueous developer. An intended use of these photoimageable polyimides is the precise placement of protective, electrically insulating coatings over conductive parts of a printed circuit pattern, followed by imagewise exposure and development to remove the coating from those parts of the circuit that provide points of connection to other circuits or electrical devices. Acidified aqueous developers offer advantages over previously discussed solvent and aqueous alkaline developers by preventing problems of copper corrosion and copper oxide formation. The use of photoimageable, aqueous acid developable polyimides distinguishes coating materials, according to the present invention, from materials using less desirable types of image developer.
More particularly the invention provides a photoimageable, aqueous acid soluble polyimide polymer comprising an anhydride, including a substituted benzophenone nucleus, a diamine reacted with the anhydride to form a photosensitive polymer intermediate, and at least 60 Mole % of solubilizing amine reacted with the photosensitive polymer intermediate to form the photoimageable, aqueous acid soluble polyimide polymer. An emulsion for electrophoretic deposition of a coating of a photoimageable, aqueous acid soluble polyimide polymer comprises a dispersed phase, including the photoimageable aqueous acid soluble polyimide polymer, dissolved in an organic solvent and a dispersion phase including a coalescence promoter and water. The emulsion may be applied, by electrophoretic deposition, to a conductive structure to provide a photoimageable coating on the conductive structure. A method for imaging a photoimageable aqueous acid soluble polyimide polymer applied to a conductive structure, used for connecting electrical or electronic components, comprises the steps of, providing a conductive structure used for connecting electrical and electronic components, and applying a coating to the conductive structure using an electrophoretic coating technique. The coating comprises an anhydride including a substituted benzophenone nucleus, a diamine reacted with the anhydride to form a photosensitive polymer intermediate, and at least 60 Mole % of a solubilizing amine reacted with the photosensitive polymer intermediate. Thereafter, exposing the coating to a pattern of radiation for photocrosslinking exposed parts of the photoimageable aqueous acid soluble polyimide polymer, and applying an aqueous acid developer solution to remove unexposed photoimageable aqueous acid soluble polyimide polymer to reveal a crosslinked polyimide polymer image of the radiation pattern.
Electrophoretic deposition techniques allow relatively precise placement of material on charged surfaces included in an electrolytic cell, operated by direct current. The charged surfaces could include suitably connected printed circuits to induce material placement on individual metal traces of the circuitry. Using electrophoretic deposition techniques, deposition of material occurs predominantly on conductive surfaces. This facilitates the coating of unsupported leads and relatively inaccessible portions of a printed circuit such as conductive traces disposed within the structure of a multilayer circuit. Traditional coating methods do not provide desirable protection for such features. In addition, precision coating via electrophoretic deposition techniques uses less material than traditional coating methods thereby providing beneficial cost savings and waste reduction. The selective placing of electrophoretically deposited films provides an added advantage, for coating flexible printed circuits, compared to blanketing layers produced with conventional coating methods. Regardless of differences in coefficient of thermal expansion, selectively deposited coatings cannot exert a force to distort the general shape of the flexible substrate material. Flexible circuits, coated using electrophoretic deposition, are lighter and less likely to exhibit cure-stress-induced curl after processing. Lower circuit weight is important for certain applications, such as interconnects for hard disk drives.
For clarification, the following definitions provide the meaning of terms that may be used throughout this specification.
The term xe2x80x9ccovercoatxe2x80x9d refers to a dielectric coating, over the basestock, applied after the conductive pattern has been etched. The basestock may be a conventional printed circuit substrate, including flexible polyimide sheet, used as a support for etched metal patterns, particularly those formed by etching copper.
The term xe2x80x9ccurrent densityxe2x80x9d means the amount of current flowing through a substrate, per unit area, perpendicular to the direction of current flow.
The term xe2x80x9ce-coatxe2x80x9d is synonymous with electrophoretic deposition and may refer herein to a coating, and technique for electrophoretically depositing such a coating.
The terms xe2x80x9cemulsionxe2x80x9d and xe2x80x9csolutionxe2x80x9d are used somewhat interchangeably to refer to polyimide containing fluids that may be understood as conventional emulsions except when suspended particles become so small that the liquid is essentially clear with little or no evidence of turbidity, i.e. its visual appearance is that of a solution. When the xe2x80x9cemulsionxe2x80x9d used for electrophoretic deposition appears to possess solution-like properties, it is considered as a solution and is so described herein.
The term xe2x80x9cunsupported leadxe2x80x9d means a conductive trace or lead that spans a void in a substrate or extends over the edge of a substrate and thereby exists in an unsupported condition.
The term xe2x80x9cmole % aminexe2x80x9d as used herein is based upon the original population of anhydride groups before reaction with a diamine to form a photosensitive polyimde moiety. For example, 60 Mole % of solubilizing amine represents an amount equivalent to 60% of the anhydride groups available in the anhydride starting material.
The xe2x80x9cpolymer intermediatexe2x80x9d refers to a reaction product, of at least two monomers, that has the capability for further reaction with other selected reactants. Anhydrides reacting with diamines, as described herein, produce polymer intermediates for further reaction with solubilizing amines.
The term xe2x80x9csolubilizing aminexe2x80x9d refers to materials containing amine functionality that may react with polymer intermediates to increase polymer solubility in solutions of aqueous acid.
The term xe2x80x9caqueous acid soluble polymerxe2x80x9d refers to a polymer that is at least partially soluble in aqueous acid solutions.
The term xe2x80x9caqueous acid developable polymerxe2x80x9d refers to a photoimageable, aqueous acid soluble polymer crosslinked by exposure to suitable radiation so that crosslinked material no longer dissolves in dilute aqueous acid. This allows dissolution of unexposed material to leave an insoluble pattern of crosslinked material corresponding to the pattern of radiation used for exposure.